Question

A force of 5 N acts on the top end of the blade of a stationary fan to rotate it anti-clockwise. At the same time, another force of 10 N acts on the middle point of the same blade to rotate it clockwise. In which direction the fan will rotate?

• A. Clockwise
• B. Anti-clockwise
• C. Clockwise followed by anti-clockwise
• D. Not rotate

Answer 1

The correct option is D Not rotate

Let the length of the blade be $r$.

Moment of force in anti-clockwise direction is the product of force and perpendicular distance to the force.

For the first force, moment of force is:
${\tau }_1=5\times r=5r$ anticlockwise
For the second force, moment of force is:
${\tau }_2=10\times r/2=5r$ clockwise

Let the anticlockwise direction of rotation be positive.
Net moment of force will be:
$\tau ={\tau }_1-{\tau }_2$
$\tau =5r-5r=0$

Hence, the fan will not rotate at all.